I need help making a Marrash a PC

[Invader]

One of my players died to a Marrash's Taklif arrow and I'm giving him the option of playing as the Marrash that rises from the original corpse.

I need help balancing the monster so he'll be the equivalent of a 4th level PC which is where the rest of the party is at.

Can everyone give me some suggestions?

The Marrash is in the MM2 page 145.

[VGLordR2]

I haven't really messed around with this at all, so I don't know for sure how balanced it is, but there is a thread where members of GiantitP submitted homebrew monster classes. Zemro made a Marrash (http://www.giantitp.com/forums/showpost.php?p=10308045&postcount=324) progression that you could easily give to your player.

[Invader]

Hmm that's not bad except all the disease stuff seems a bit clunky but I could always just get rid of that stuff.